The present invention relates to a bearing assembly for a worm drive comprising thrust bearings and radial bearings supporting the rotary shaft of the worm of the worm drive, especially to a bearing assembly for a worm drive that is applicable to a seat position adjustment mechanism of an automobile power seat.
A power seat is known wherein position adjustment of each part of a reclining seat is made electrically. A seat slide mechanism of an automobile power seat comprises a seat bottom fixed through a frame on slide rails that are fixed to the floor of the vehicle, a seat back extending upward from the rear end of the seat bottom, and an electric motor mounted on the bottom of the frame for driving this frame back and forth.
The electric motor is fixed on a lower portion of the frame. To the drive shaft of this electric motor, the proximal end of a double wire comprising an inner wire and a pipe covering the inner wire in sliding contact therewith is connected. The distal end of the double wire is connected to a rotatable screw shaft through a worm drive so that driving force of the electric motor is transmitted to the screw shaft through the double wire and the worm drive.
The slide rails comprise a pair of lower rails arranged in the widthwise direction and integrally fixed on the floor and a pair of upper rails arranged in the widthwise direction over the respective lower rails and guided by the lower rails so as to be movable in the longitudinal direction. The frame is bridged between the pair of upper rails. The seat bottom is fixed on the upper rails and the frame, thereby being movable in the longitudinal direction while guided by the lower rails through the upper rails.
One of the upper rails has an inverted U-shaped cross section, in which the screw shaft extending in the longitudinal direction is mounted. The screw shaft is in threaded engagement with a nut member mounted in the lower rail so as to be rotatable relative to the nut member. Thus the upper rails can be moved back and forth with forward and reverse rotations of the nut member so as to adjust the longitudinal position of the power seat.
Thus, responding to the requirements of power- and weight-saving, the seat position adjustment mechanism of an automobile power seat is designed so that its drive shaft rotates with lower torque so as to be driven with a small motor, as well as to save weight. In such a design, the worm drive used in a seat position adjustment mechanism of an automobile power seat is required to be with lower rotational torque so as to always move smoothly with lower drive force as well as to be lightweight. This worm drive is a mechanism comprising a worm (screw gear) and a corresponding worm wheel (helical gear). It is used in a seat position adjustment mechanism of an automobile power seat since its reduction ratio in one step is large and backlash is small compared with other gear mechanisms.
For example, a bearing assembly for a worm drive wherein sintered bushes are used as radial bearings of the worm is simple in structure and lightweight compared to one wherein rolling bearings are used.
Also, weight can be saved by forming thrust bearings for the worm with a resin.
However, since load from the worm wheel works to the worm as axial load because of interlock of gears during rotation, the rotary shaft of the worm is prone to displacement in the axial direction. This makes the thrust bearings of the worm prone to wear from the received load, which may lead to vibration or noise from wear. To prevent this, wear resistance is required for the thrust bearings. Therefore, in many cases, as thrust bearings for the worm, metal rolling bearings were used which also serve as radial bearings. This prevented weight saving.
A known slide bearing 20 for a worm shown in FIG. 3 is a truncated conical member through which a bearing hole extends. It is a slide bearing serving as both thrust and radial bearings for the worm 22.
However, the above-mentioned conventional bearing 20 for a worm is pressed strongly against a tapered support hole of a bracket by spring force of a metal wave washer 21. The bearing 20 is therefore prone to wear and it is difficult to make its sliding rotation constantly smooth.
As described above, because the worm 22 meshes with the worm wheel 23, load from the worm wheel 23 is applied to the worm 22 in its axial direction. The load is thus applied to the rotary shaft of the worm 22 every time when direction or rate of rotation changes, and the slide bearing 20 for the worm is thus prone to wear. As a result, transmission efficiency is impaired and problem of vibration and noise occurs.
Also, if a rolling bearing is used as a thrust bearing in a worm bearing assembly to improve durability, weight saving requirement is not fulfilled.
The object of this invention is therefore to solve the problems described above, saving weight of a thrust bearing in a worm drive as well as reducing wear of the thrust bearing, thereby preventing deterioration of transmission efficiency and occurrence of vibration or noise, obtaining a bearing assembly for a worm drive with excellent duration, especially suitable for use in a seat sliding mechanism for an automobile power seat.